1. Field of the Invention
The present invention relates to a wiring board, such as a flexible wiring board, and a method of manufacturing the same.
2. Discussion of the Prior Art
A conventional wiring board, such as a flexible wiring board, is structured so that a circuit pattern made of conductive material, for example, copper, is layered over an insulating substrate made of high polymer.
There are generally known three methods of manufacturing the wiring board: 1) substractive method, 2) additive method, and 3) semiadditive method. In the substractive method, an adhesive layer is layered on the surface of an insulating substrate, and a conductive thin film is layered on the adhesive layer. A etching resist layer with a prescribed circuit pattern is layered on the surface of the conductive thin film. The portion of the etching resist layer which is not masked is etched away, so that the necessary circuit pattern is left or formed on the insulating substrate. In the additive method, conductive material is deposited on and fixed to only the portions of a designed circuit pattern on an insulating substrate by a plating method, for example, thereby forming the circuit pattern on the insulating substrate. The semiadditive method is the combination of the substractive method and the additive method.
The wiring board is therefore formed by combining the insulating substrate and the circuit pattern, which are both made of different materials, into a one piece construction. The expansion and contraction coefficients of the wiring board are different from those of the circuit pattern. Due to the coefficient difference, warpage of the layers and separation of one layer from the other will occur when it is manufactured or used. When the wiring board is repeatedly used, the circuit pattern will be cut off due to fatigue of the metal. Dust is likely to collect on the surface of the wiring board because it contains the circuit pattern, and is, hence, uneven. Dielectric breakdown will take place in the circuit pattern. The relatively heavy weight of the circuit pattern, which is made of metal, hinders the reduction of weight of the wiring board. Use of the adhesive layer of low heat resistance, which is for bonding the circuit pattern to the insulating substrate, limits the heat resistance of the wiring board to be low.
All of the known manufacturing methods, described above, require a number of manufacturing steps, thus hindering manufacturing efficiency and a reduction of manufacturing costs.
To solve the problems of the conventional wiring board and the method of manufacturing the same, the Applicant of the present Patent Application proposed a new technique set forth in Japanese Patent Application No. Hei. 1-4125. In the proposed technique, the surface of an insulating substrate is selectively radiated with an ion beam to render conductive the portions of the substrate irradiated with the ion beam. The resultant wiring board takes the integral form of the circuit pattern and the insulating substrate.
A typical process to realize the technique is illustrated in FIG. 9(a)(PRIOR ART). As shown in FIG. 9(a), a circuit pattern mask 3 made of metal foil and having through-holes 31 formed corresponding to a circuit pattern is laid over an insulating substrate 1 made of high polymer. The surface of the insulating substrate 1 on which the circuit pattern mask 3 is laid is irradiated with ion beams. Only the portions of the insulating substrate 1 which are exposed through the through-holes 31 to ion beams are rendered conductive. Consequently, a wiring board having a circuit pattern 2, which corresponds to the circuit pattern in the surface region, is formed, as shown in FIG. 9(b)(PRIOR ART).
In the wiring board thus manufactured, at the junctions between the circuit pattern 2 and the insulating substrate 1, mixing and carbonization of substrate 1 progresses due to the ion irradiation. Each junction takes the form of a continuous phase between the different materials of the circuit pattern and the insulating substrate. Thus, the wiring board is a single uniform sheet. Therefore, the circuit pattern and the insulating substrate will not be warped and separated from each other, and dust will not collect thereon. Further, not using a metal conductor reduces the weight of the wiring board, and not using an adhesive allows the heat resistance of the wiring board to increase. In the technique proposed by the Applicant, an extended period of time is required to radiate the ion beams in order to form the circuit pattern 2. As described in the description of the preferred embodiment of the gazette, Japanese Patent Application No. Hei. 1-4125, to make polyetherimide conductive until its volume specific resistance is 3.times.10.sup.3 .OMEGA.cm, a B.sup.+ ion must be applied to the polyetherimide under the condition of 1.0 MV of an accelerating electric field and 3.times.10.sup.17 ;/cm.sup.2 of dosage. Therefore, it takes 7 hours for the irradiation by 2.0 .mu.A of beam current.
High conductivity of a metal may be utilized to increase the conductivity of the circuit pattern to be formed in the insulating substrate by irradiating the insulating substrate with ions of metal, such as copper, chromium, and nickel. In this case, at least a 3.times.10.sup..intg. /cm.sup.2 dosage is required in order that metal atoms stay in the insulating substrate to allow the high conductivity of metal to be utilized. Generally, an ion-irradiating apparatus can generate a smaller dosage of metal ions than that of ions of gas, such as nitrogen and oxygen. Therefore, the technique of using the metal ions also takes a long processing time to form the circuit pattern.
As described above, the technique to form the circuit pattern in the insulating substrate by selectively irradiating the surface of the insulating substrate with ion beams takes a long time to form the insulating substrate. In this respect, the technique is poor in production efficiency. Furthermore, since the insulating board is made of high polymer, it is not suitable for a situation in which it is used for a long time at high temperature.